Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2017		$ 882.0	$ 408.6	$ 1,047.3	$ 9,685.1	$ (414.3)	$ (1,392.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Options and Awards				(83.5)			102.5
Stock Options and Awards — Amortization				56.6
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income					25.3	(25.3)
Net Income	$ 381.6				381.6
Dividends Declared — Common Stock					(96.4)
Dividends Declared — Preferred Stock					(17.3)
Net Unrealized (Losses) Gains on Debt Securities Available for Sale	(55.0)					(55.0)
Net Unrealized (Losses) Gains on Cash Flow Hedges	(3.7)					(3.7)
Foreign Currency Translation Adjustments	(13.1)					(13.1)
Pension and Other Postretirement Benefit Adjustments	5.7					5.7
Stock Purchased							(263.2)
Ending balance at Mar. 31, 2018	10,226.0	882.0	408.6	1,020.4	9,973.8	(505.7)	(1,553.1)
Beginning balance at Dec. 31, 2018	10,508.3	882.0	408.6	1,068.4	10,776.8	(453.7)	(2,173.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Options and Awards				(120.4)			126.0
Stock Options and Awards — Amortization				55.3
Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income					0.0	0.0
Net Income	347.1				347.1
Dividends Declared — Common Stock					(133.7)
Dividends Declared — Preferred Stock					(17.3)
Net Unrealized (Losses) Gains on Debt Securities Available for Sale	85.6					85.6
Net Unrealized (Losses) Gains on Cash Flow Hedges	(4.3)					(4.3)
Foreign Currency Translation Adjustments	14.8					14.8
Pension and Other Postretirement Benefit Adjustments	12.3					12.3
Stock Purchased	(257.4)						(257.4)
Ending balance at Mar. 31, 2019	$ 10,616.2	$ 882.0	$ 408.6	$ 1,003.3	$ 10,972.9	$ (345.3)	$ (2,305.3)